The present invention relates to the nucleotide sequence of a glutamine synthetase from sugarbeet, to a vector comprising this nucleotide sequence, to cells which are transformed with this vector, to proteins encoded by this nucleotide sequence, to plants which have been transformed with this nucleotide sequence, and to methods for the genetic modification of plants, in particular sugarbeet.
The accumulation of glutamine as the main xcex1-amino-N component which occurs in the storage root, that is to say the storage organ of the sugarbeet, and which is also referred to as harmful nitrogen, gives rise to considerable problems in sugar production. The compensation, generally by rendering alkaline, for the acidification of the beet juice which is caused,by this component is costly, leads to faster wear on the production systems and, finally, also involves considerable environmental pollution, which can likewise be prevented only by use of costly measures. This glutamine is synthesized in the plant through amidation of glutamic acid by NH4+ being bound, with consumption of ATP, to C-4 of glutamate. This step is catalyzed by the enzyme glutamine synthetase (abbreviated to GS hereinafter). This enzyme is present both in the chloroplast and in the cytoplasm of the plant cell. In the chloroplast, the enzyme occurs as a tetramer which is encoded by one gene and consists of up to five subunits (GS-2). In the cytoplasm, according to current knowledge, the enzyme normally occurs as a heterooligomer (GS-1) encoded by more than one gene. The various GS-1 isoenzymes known are usually heterooctamers. Six different isoforms of GS-1 have been found to date. The glutamine synthetase localized in the chloroplast, that is to say GS-2, has the main function of binding the NH4+ produced during photorespiration and converting the NH4+ derived from nitrate reduction into organic compounds. The function of GS-1 is, by contrast, mainly in catabolic degradation pathways during the course of which the NH4+ resulting from protein degradation is fixed. Such degradation pathways are particularly important during aging of the leaf, that is to say during senescence, and the resulting glutamine is exported from the leaf into storage organs.
In contrast to GS-2 in the chloroplast, relatively little is known about GS-1 in the cytoplasm. This relates in particular to its function in the cell, but also to its regulation. In contrast to GS-2, it has been assumed to date that GS-1 is encoded by more than one gene, the number thereof being variable. The genes show homologies with one another but can be unambiguously separated from one another (Brears et al., Plant J. 1 (1991), 235 to 244; Edwards et at al., Plant Cell 1 (1989), 241 to 248). In addition, the GS-1 genes, which each encode one subunit of the octameric holoenzyme, appear to be regulated differently (Petermann and Goodman MGG 230 (1991), 145 to 154). Controlled external influencing of glutamine metabolism by modification of GS-1 is made difficult thereby.
The localization of GS-1 in the plant is also still substantially unknown. It is known from Edwards et al. (loc. cit.) that one isoform is thought to be expressed exclusively in the phloem, where it possibly plays a part in intercellular transport. Sakurai et al. (Planta 200 (1996), 306 to 311) reports that in rice one isoform of GS-1 is present in the conducting bundles and evidently plays a part in the export of glutamine from leaves. It is also known that tobacco and alfalfa plants which have been transformed with a GS-1 gene from Lotus corniculatus show expression in the flowers (Carrayol et al., Plant Sci 125 (1997), 75 to 85). It is additionally known that the composition and localization of GS-1 holoenzymes in the root nodules of Lotus corniculatus may vary greatly.
No targeted reduction, carried out by methods of molecular biology, in the glutamine content in plants, especially in storage organs of plants, has been disclosed to date. The essential difficulties occurring in the targeted reduction of the glutamine synthetase activity in plants and the eventually desired reduction in the nitrogen content in storage organs of plants derive from the fact that the glutamine synthetase activity must be restricted tissue- and time-specifically in such a way that the nitrogen content in the target organ, for example the storage root of a sugarbeet, is reduced. This must not involve any impairment of the other functions and properties of the sugarbeet. On the contrary, it must be ensured that the overall physiology of the sugarbeet remains intact and there is merely a specific reduction in the nitrogen content in the storage organ of the sugarbeet. Because of the problems described concerning the localization of GS-1 and the lack of clarity in relation to the time specificity of its activity, no successful experiments have been disclosed indicating that it was possible to reduce the nitrogen content in storage organs of sugarbeet via modulation of the GS-1 activity.
The technical problem on which the present invention is based is thus to provide means and methods which make it possible in a targeted manner, that is to say tissue- and time-specifically, to reduce the nitrogen content in the storage organ of a plant, in particular of a sugarbeet, without this involving impairment of the vital, growth and reproductive functions of the sugarbeet and its commercial value.
The present invention solves this problem by providing an isolated and purified nucleotide sequence and vectors comprising this nucleotide sequence, which code for one subunit of the GS-1 of sugarbeet. The present invention solves the technical problem also by providing the isolated and purified protein encoded by this nucleotide sequence, in particular the amino acid sequence of the GS-1 subunit from sugarbeet, and by methods for the genetic modification of plants, in particular sugarbeet, where the content of glutamine synthetase in a plant, in particular in its senescent leaves, is altered, in particular reduced, by transforming cells of this plant with said vectors, and regenerating from the transformed cells intact, propagatable, stably transformed transgenic plants in whose senescent leaves the activity of GS-1 is reduced or completely suppressed.
The invention is surprising and advantageous in particular because the subunit encoded by the nucleotide sequences of the invention represents the only subunit of the oligomeric GS-1 isoform occurring in the senescent leaves of the sugarbeet. Accordingly, the invention also provides the surprising information that this isoform of GS-1 is a homooctamer. This makes it possible in a surprising manner to influence the activity of this enzyme by influencing the expression of a single gene, namely the gene encoding the GS-1 subunit, in particular to prevent or reduce the expression thereof. The invention is also surprising inasmuch as the homooligomeric isoform of GS-1 provided by the invention occurs only in the stage of senescence and accordingly displays, besides the tissue specificity in relation to the localization in the leaf which has been mentioned above, also a time specificity in relation to the occurrence during senescence. The present invention surprisingly therefore provides means and methods for influencing the qualitative and/or quantitative occurrence of a homooctameric GS-1 isoform which can be found only in senescent leaves of the sugarbeet. The nucleotide sequences of the invention can also be used for cloning homologous genes, in particular the coding regions thereof, in other tissues and even other plants and organisms. It is possible in particular on use of the present nucleotide sequence as hybridization probe in homologous or heterologous systems also to identify and isolate endogenous regulatory noncoding nucleotide sequences which are associated with this sequence and which, for example, mediate time- and tissue-specific expression.
The invention solves the present technical problem in particular by providing a nucleotide sequence for modulating the expression, in particular for suppressing the expression, of a protein having the activity of a glutamine synthetase, in particular the activity of a GS-1, which is selected from the group consisting of
a) the DNA sequence of SEQ ID No. 1, 3 or a part thereof,
b) a nucleotide sequence which encodes the amino acid sequence of SEQ ID No. 2 or a part thereof,
c) a nucleotide sequence which is complementary to the nucleotide sequences of a) or b), or a part thereof, and
d) a nucleotide sequence which hybridizes with the nucleotide sequences of a) to c).
The nucleotide sequence of the invention is functionally characterized in that, in a cell transformed therewith and having an endogenous GS-1-encoding sequence, it modulates the activity of the GS-1 activity of this transformed cell, for example increases the GS-1 activity, for example by overexpression, or reduces or completely suppresses the GS-1 activity, for example through the nucleotide sequence of the invention being transformed in the form of an antisense construct which inhibits endogenous GS-1 translation.
The invention provides in a particularly preferred embodiment for the nucleotide sequence to be derived from the sugarbeet Beta vulgaris. 
The nucleotide sequence of the invention may be a DNA sequence, for example a genomic, where appropriate. intron-interrupted DNA sequence or CDNA sequence, but it can also be an RNA sequence, for example an mRNA sequence or synthetically prepared. The present invention relates both to the sense and to the antisense nucleotide sequences. The nucleotide sequences of the invention can be so-called full-length sequences, that is to say sequences which encode a complete protein having the activity of a glutamine synthetase, in particular of the GS-1 of sugarbeet, where appropriate including the translation initiation site. However, the invention also relates to partial sequences of such nucleotide sequences, in particular those which serve to modulate the expression of the protein having the activity of a glutamine synthetase, in particular of a GS-1 from sugarbeet. Accordingly, the nucleotide sequence of the invention may also form a fusion gene in a transcription or translation unit with other nucleotide sequences. The invention relates in an advantageous refinement also to nucleotide sequences which hybridize with the DNA sequence specified in SEQ ID No. 1 or 3 or hybridize with a nucleotide sequence which encodes the amino acid sequence of SEQ ID No. 2, and nucleotide sequences which hybridize with nucleotide sequences complementary tothe two sequences mentioned.
In connection with the present invention, hybridization means a prehybridization, a hybridization and subsequent washing. The prehybridization preferably takes place in an aqueous solution composed of 6xc3x97SSPE, 0.1% SDS and 5xc3x97Denhardt""s reagent, and 500 xcexcg/ml denatured herring sperm at 60xc2x0 C., particularly preferably at 65xc2x0 C., for 3 hours. The hybridization preferably takes place in an aqueous solution composed of 3xc3x97SSPE, 0.1% SDS, 5xc3x97Denhardt""s reagent and 500 xcexcg/ml denatured herring sperm at 60xc2x0 C., particularly preferably at 65xc2x0 C., for 16 hours. The washing preferably takes place in an aqueous solution composed of 2xc3x97SSPE and 0.1% SDS at room temperature for 10 minutes, this being followed by another washing step in an aqueous solution composed of 2xc3x97SSPE and 0.1% SDS at 60xc2x0 C., particularly preferably at 65xc2x0 C., for 15 minutes and by a final washing step with an aqueous solution composed of 0.4xc3x97SSPE and 0.02% SDS at 60xc2x0 C., particularly preferably at 65xc2x0 C., for 15 minutes.
In a particularly preferred embodiment, a prehybridization is carried out in an aqueous solution composed of 6xc3x97SSPE, 0.1% SDS and 5xc3x97Denhardt""s reagent, and 500 xcexcg/ml denatured herring sperm at 65xc2x0 C. for 3 hours. The hybridization takes place in an aqueous solution composed of 3xc3x97SSPE, 0.1% SDS, 5xc3x97Denhardt""s reagent and 500 xcexcg/ml denatured herring sperm at 68xc2x0 C. for 16 hours. The washing takes place in an aqueous solution composed of 2xc3x97SSPE and 0.1% SDS at 68xc2x0 C. for 15 minutes, this being followed by another washing step in an aqueous solution composed of 1xc3x97SSPE and 0.1% SDS at 68xc2x0 C. for 15 minutes and by a final washing step with an aqueous solution composed of 0.1xc3x97SSPE and 0.1% SDS at 68xc2x0 C. for 15 minutes.
The present invention also relates, of course, to modifications of the aforementioned sequences, in particular those which display, by comparision with the sequences shown in SEQ ID No. 1 or 3, nucleotide additions, deletions, inversions, substitutions or the like, including chemical derivatizations or replacement, exchange or addition of unusual nucleotides.
The invention also relates to nucleotide sequences which have a degree of homology of at least 80%, preferably 90%, to the sequences shown in SEQ ID No. 1 or 3.
The nucleotide sequences of the present invention are advantageous in particular inasmuch as they serve to modulate the expression of a protein having the activity of a glutamine synthetase, in particular the GS-1 from sugarbeet. The nucleotide sequences of the invention can be employed for altering, in particular reducing, and in a particularly preferred manner completely suppressing, the amount of glutamine synthetase formed, in particular the GS-1 from sugarbeet. In a particularly preferred manner, the invention makes it possible through modulation of the expression in a tissue- and time-specific manner for the deposition of glutamine in the storage organ of the sugarbeet to be suppressed without this inevitably entailing the need to employ tissue- and time-specific regulatory elements for the transgene, that is to say the nucleotide sequence of the invention. This is because the nucleotide sequences of the invention encode the GS-1 of sugarbeet, which occurs specifically only in senescent leaves, and accordingly shows site specificity in relation to the expression in leaves and time specificity in relation to the expression during senescence. The nucleotide sequences of the invention make it possible to inhibit this glutamine synthetase, which occurs specifically at leaf senescence, by means of a single gene construct because the GS-1 of sugarbeet is a homooctamer and accordingly the formation of all the GS-1 subunits can be switched off by means of a single gene construct. Accordingly, the invention provides in a particularly preferred manner for the use of antisense constructs which specifically suppress the formation of protein, that is to say GS-1, in the senescent leaves of sugarbeet. It is possible by use of antisense constructs to inhibit the expression of the GS-1 of sugarbeet which occurs specifically at leaf senescence, so that glutamine formation and deposition of glutamine in the storage root is prevented. The growth of the sugarbeet is advantageously not impaired in this process because GS-2 is not affected by the genetic manipulation of the plant cell.
In connection with the present invention, the activity of a protein having the activity of a glutamine synthetase means an activity by which NH4+ is bound enzymatically to C-4 of a glutamate molecule with use of ATP.
In connection with the present invention, modulation of the expression of a protein means a deliberate change, that is to say increase or reduction, achieved by genetic engineering methods, in the amount of protein in a cell compared with the amount of protein naturally present in the relevant cell at the relevant time.
The modulation of expression can take place by influencing the translation or transcription of the endogenous nucleotide sequence encoding the protein, for example by introducing an antisense construct which partially or completely reduces the amount of translatable mRNA. An increase in the amount of protein may take place, for example, by introducing a nucleotide sequence which encodes the protein and is under the control of overexpressing regulatory elements, or by introducing multiple gene copies.
Further alternative or additional modulations can be achieved by employing tissue- or time-specific, inducible or constitutively expressed regulatory elements which lead to an expression pattern which is altered by comparison with the natural expression pattern in the relevant cell and at the relevant time or at the relevant stage of development of the cell or the plant.
The present invention relates in a further embodiment to vectors comprising at least one of the nucleotide sequences of the invention. In a particularly preferred embodiment of the invention, such a vector is embodied as plasmid or viral vector.
The present invention also relates to vectors of the aforementioned type, where the at least one nucleotide sequence of the present invention is under the control of regulatory nucleotide sequences which are likewise present in the vector and which are arranged, for example, 5xe2x80x2, 3xe2x80x2, 5xe2x80x2 and 3xe2x80x2 or else within the nucleotide sequence. These regulatory nucleotide sequences may be heterologous to the nucleotide sequence of the invention, that is to say be derived from a different organism or from a different gene, or homologous, that is to say also naturally occurring together with the nucleotide sequences of the invention in a regulatory unit.
The invention accordingly also relates to vectors of the aforementioned type, where a nucleotide sequence controlling the expression of the nucleotide sequence of the invention, in particular a promoter, is located 5xe2x80x2-wards of the nucleotide sequence of the invention. In a particularly preferred embodiment of the invention, the promoter is the 35 S promoter of CaMV or a promoter of the T-DNA of Agrobacterium tumefaciens, for example the promoter of the nopaline synthetase or octopine synthetase gene.
The invention provides in a further embodiment for a transcription termination unit, in particular a 3xe2x80x2-polyadenylation signal, to be located 3xe2x80x2-wards of the nucleotide sequence of the invention, particularly preferably the polyadenylation signal of the NOS gene of Agrobacterium tumefaciens. 
The present invention provides in another preferred embodiment for the regulatory sequences to be inducible, for example by external factors.
The invention provides in another preferred embodiment for the regulatory sequences of the expression of the nucleotide sequences of the invention controlled thereby to confer tissue specificity and/or timespecificity, for example to bring about expression of an antisense construct specifically in leaves during senescence.
The invention also provides for the nucleotide sequences of the invention, where appropriate in a unit with the regulatory nucleotide sequences assigned to them, to be arranged in the vector together with nucleotide sequences which assist transfer and integration or recombination of the nucleotide sequences of the invention where appropriate with the regulatory nucleotide sequences assigned to them into the genome of a transformed cell. The nucleotide sequences of the invention can therefore be arranged, for example, between the left and right border region, flanked by only one border region in each case and/or interrupted by one or more border regions of Agrobacterium tumefaciens or Agrobacterium rhizogenes. 
The present invention also relates to cells comprising at least one of the aforementioned vectors. In a particularly preferred manner, such cells are bacterial cells, yeast cells or plant cells, in particular plant cells from monocotyledonous or dicotyledonous plants, in particular sugarbeet. The present invention therefore particularly relates to a non-variety-specific cell of a plant which has been transiently or stably transformed with a nucleotide sequence of the invention, in particular which has this nucleotide sequence in its genome, for example in the form of an antisense construct. A plant means a photosynthetically active organism including algae, mosses, ferns and higher plants.
The invention also relates to cell assemblages, tissues, organs, parts of organs, cell cultures, calli, differentiated or undifferentiated cell aggregates, embryos, protoplasts etc. of an organism which have, stably integrated into the genome or transiently present, at least one cell transformed with the nucleotide sequences of the invention. In a particularly preferred manner, the invention relates to leaves, stalks, seeds, roots, storage organs, petals, flower organs etc. of a plant, said organs having at least one cell stably or transiently transformed with the nucleotide sequences of the invention. In a particularly preferred manner, the plants or the parts thereof are transformed in such a way that the transformed nucleotide sequence is stably inherited from generation to generation.
The present invention relates not only to cells, cell assemblages, calli and plant organs but also, of course, to plants, in particular intact fertile plants which have been transformed by means of the nucleotide sequence of the invention and have in at least one of their cells at least one of these sequences, in particular stably integrated in their genome. The transformation preferably takes place, as stated hereinafter, nonbiologically, where agrobacterium-mediated gene transfer is understood to be nonbiological. The resulting cells, and the plant tissues, plant organs, plant parts or plants having these cells, are not variety-specific. On the contrary, the invention is applicable to virtually all plants, plant families or plant genera.
In a particularly preferred manner, the transformed nucleotide sequence is heterologous to the transformed cell, that is to say is not naturally present in the transformed cell, is not present in the artificially generated high copy number, or is not expressed at the place or time at which it is expressed according to the invention. In cases in which the cell to be transformed already has an endogenous identical or similar nucleotide sequence, the cell obtained by the transformation of the invention differs from the initial cell for example in that the introduced nucleotide sequence is present in a different genetic context in the genome, has different regulatory elements, is arranged in antisense orientation to its regulatory elements and/or is present in increased copy number.
The invention accordingly also relates to methods for producing transgenic cells, where the nucleotide sequence of the invention to be transformed is introduced into the cell to be transformed by means of a conventional transformation method, for example microprojectile bombardment, agrobacterium-mediated gene transfer, electroporation, PEG-mediated transformation or the like.
The invention also relates to methods for producing transgenic plants having the nucleotide sequences of the invention, where cells or cell assemblages transformed with the nucleotide sequences of the invention are cultivated and regenerated to intact, preferably fertile, plants. The cultivation and regeneration take place by conventional methods.
The invention also relates to a protein having the activity of a glutamine synthetase, in particular the GS-1 from sugarbeet, the latter being encoded by the nucleotide sequences of the invention, in particular the nucleotide sequence shown in SEQ ID No. 1, particularly preferably an amino acid sequence shown in SEQ ID No. 2. The invention also relates to proteins having the activity of a glutamine synthetase, in particular the GS-1 from sugarbeet, this sequence having modifications such as amino acid exchanges, deletions, additions, inversions or the like, and the protein having the activity of a GS-1 from sugarbeet. The invention also relates to proteins which, at the amino acid level, have a degree of homology (identical amino acids) of at least 90%, preferably 95%, with the sequence shown in SEQ ID No. 2. Proteins of this type can be prepared by employing the nucleotide sequences of the invention as cloning probes or hybridization probes for identifying and cloning homologous genes encoding these proteins.
The invention relates in a further embodiment to monoclonal or polyclonal antibodies against one, of the aforementioned proteins, these antibodies recognizing and binding one or more epitopes of said proteins.
The invention relates in a further embodiment to methods for altering glutamine metabolism in a sugarbeet, in particular for modulating the expression, particularly preferably for repressing, a protein having the activity of a glutamine synthetase, in particular the GS-1 from sugarbeet, where the glutamine synthetase content of the sugarbeet is altered by transforming at least one sugarbeet cell with a vector of the present invention, in particular transforming with a vector having the nucleotide sequence of the invention in antisense orientation, and regenerating a sugarbeet from the transformed cell or an association of cells. A sugarbeet generated in this way is advantageously characterized in that the glutamine synthetase GS-1 normally formed in its leaves during senescence is not formed because the expression of glutamine synthetase GS-1 is prevented because of the antisense construct present at least in the leaves and expressed there, so that the formation of glutamine in the leaves and, eventually, the deposition of the glutamine in the storage root is prevented.
However, the invention also of course relates to methods for altering glutamine metabolism in plants, in particular sugarbeet, according to which the glutamine synthetase content in particular cells or organs is increased, where appropriate at certain times, in particular by transforming gene constructs which make constitutive and/or enhanced expression of the nucleotide sequences of the invention possible.
Further advantageous refinements of the invention are evident from the dependent claims.
The invention is explained in detail by means of examples and drawings belonging thereto.
SEQ ID No. 1 shows the translated region of the cDNA sequence of the GS-1 from sugarbeet.
SEQ ID No. 2 shows the amino acid sequence of the GS-1 from sugarbeet.
SEQ ID No. 3 shows the complete cDNA sequence of the GS-1 from sugarbeet.